Solution.java

public class Solution {
    public void solve(char[][] board) {
        if (board == null || board.length == 0 || board[0].length == 0) {
            return;
        }
        int m = board.length;
        int n = board[0].length;
        //Step 1 : Flip 'O' to '*' which can never be captured.        
        //Start with the edges as 'O''s on the edges can never be captured.        
        for (int i = 0; i < n; i++) {
            if (board[0][i] == 'O') {
                dfs(0, i, board, m, n);
            }
            if (board[m - 1][i] == 'O') {
                dfs(m - 1, i, board, m, n);
            }
        }
        for (int i = 0; i < m; i++) {
            if (board[i][0] == 'O') {
                dfs(i, 0, board, m, n);
            }
            if (board[i][n - 1] == 'O') {
                dfs(i, n - 1, board, m, n);
            }
        }
        //Step2 : Flip all 'O' to 'X' since the remaining 'O''s can be captured.        
        //Also flip '*' to 'O''s       
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == 'O') {
                    board[i][j] = 'X';
                }
                if (board[i][j] == '*') {
                    board[i][j] = 'O';
                }
            }
        }
    }

    private void dfs(int i, int j, char[][] board, int m, int n) {
        if (i < 0 || i >= m || j < 0 || j >= n) return;
        board[i][j] = '*';
        if (i > 1 && board[i - 1][j] == 'O') {
            dfs(i - 1, j, board, m, n);
        }
        if (i < m - 1 && board[i + 1][j] == 'O') {
            dfs(i + 1, j, board, m, n);
        }
        if (j > 1 && board[i][j - 1] == 'O') {
            dfs(i, j - 1, board, m, n);
        }
        if (j < n - 1 && board[i][j + 1] == 'O') {
            dfs(i, j + 1, board, m, n);
        }
    }
}